The blood-brain barrier (BBB) protects and regulates the homeostasis of the brain and prevents the free passage of molecules into most parts of the brain, thereby limiting the treatment of many brain diseases. Transport of essential molecules such as nutrients, growth factors and hormones is achieved via a series of specific transporters and receptors that regulate passage across the brain endothelial cells. The delivery of biologics, and other drugs, to the brain therefore represents a significant challenge. Additionally, transport mechanisms appear to exist that rapidly remove antibodies from the brain presumably to prevent inflammatory responses due to engagement of Fc with effector ligands that promote a proinflammatory response.
Over the last decade reports of antibody transport across the blood brain barrier have emerged where binding to the extracellular domain of the transporter molecules facilitates transcytosis of the receptor antibody complex across the endothelial cell layer.
The blood-brain barrier is mainly formed by brain capillary endothelial cells (BCECs) (Rubin & Staddon, Ann. Rev. Neurosci. 1999; 22:11-28), although other cell types, such as pericytes, astrocytes and neuronal cells, also play an important role in the function of the BBB. BCECs have specific characteristics, such as tight junctions, which prevent paracellular transport of small and large (water soluble) compounds from the blood to the brain (Brightman & Reese, J. Cell Biol. 1969; 40(3):648-77; Reese & Karnovsky, J. Cell Biol. 1967; 34(1):207-17). The BBB functions as a physical, metabolic and immunological barrier (Gaillard et al., Microvasc. Res. 2003; 65(1):24-31).
A single domain antibody (Llama) called FC5 that is able to cross the BBB was identified by an academic lab (Muruganandam et al., FASEB 2001; Abulrob et al., J. Neurochemistry 2005; 95:1201-1214; PCT Publication No. WO 02/057445 A1, U.S. Pat. No. 8,383,107) using human brain endothelial cell surface selections. FC5 is species cross reactive and can bind to brain endothelial cells of rat, mouse, cyno and human. The putative receptor for this antibody is TMEM30a, also referred to as transmembrane protein 30A, cdc50a, FLJ10856, or C6orf67. This is an orphan receptor where the exact function is unknown. It is thought to play a role in aminophospholipid translocation where the complex is composed of at least two proteins: an alpha subunit from the P4 subfamily of P-type ATPases and a beta subunit from the CDC50-Lemp3 family which includes TMEM30a (cdc50a) (Chen et al., J. Immunol. 2011; 186:3215-3225; Munoz-Martinez et al., Biochemical Pharmacology 2010; 80:793-800).